1. Field of the Invention
The present invention relates to an organic electrolytic solution and a lithium-sulfur battery comprising the same, and more particularly, to an organic electrolytic solution comprising a phosphine sulfide-based compound that can improve discharge capacity and cycle life of a lithium-sulfur battery, and a lithium-sulfur battery comprising the same.
2. Description of the Related Art
Demands for secondary batteries are increased according to rapid progress in portable electronic devices, and a battery having high energy density that can meet the present trend of light, thin, short and small dimensions in portable electronic devices is continuously required. A battery that can be cheap and safe and provide environment-affinitive aspect is required to meet such need.
The lithium-sulfur battery is most promising in energy density among the batteries that have ever been developed. The energy density of lithium is 3830 mAh/g, and the energy density of sulfur (S8) is 1675 mAh/g. An active material used therein is cheap itself and environment-affinitive; however, such battery system has not been commercialized yet.
The lithium-sulfur battery cannot be commercialized because the ratio of the amount of sulfur participating in electrochemical oxidation-reduction reaction to the amount of sulfur contained within the battery is so low that the battery shows low battery capacity.
For the lithium-sulfur battery, elemental sulfur is used as an initial anode active material. As the discharge of battery proceeds, eight sulfur atoms in cyclic molecular states are changed to linear molecular states while being reduced, and finally changed to S2− ionic states by continued reduction. The resulting S2− bonds chemically to surrounding lithium cations to form lithium sulfide (Li2S). Since the resulting lithium sulfide precipitates on an anode surface to reduce the activated area of the battery, and it cannot be oxidized during charging, the battery capacity is decreased. Accordingly, it is necessary to dissociate such lithium sulfide to maintain the activated area of the battery.
Approaches to solve such problems have been tried as follows. U.S. Pat. No. 6,030,720 uses the solvent including R1(CH2CH2O)nR2 as a main solvent, in which n is 2 to 10, R1 and R2 are the same or different from each other, and represent a substituted or unsubstituted alkyl or alkoxy group, and a crown ether or a cryptand as a cosolvent. A donor or an acceptor cosolvent is included wherein the donor solvent has a donor number of at least 15. The separation distance of the battery must be 400 μm or less.
In general, the formation and precipitation of Li2S on the surface of electrodes when a lithium-sulfur battery is discharged are known as a major cause of a drop in battery capacity. Research has been conducted to increase the capacity of the lithium-sulfur battery. In most cases, ether-based solvents capable of stabilizing the lithium-sulfides are used, and an initial discharge capacity is about 840 mAh/g-sulfur, which is about 50% of the theoretical capacity. The polar solvents such as DMF, DMAc, etc. were tried to dissociate Li2S, but the polar solvents vigorously react with lithium, and thus the polar solvents can not be applied to the lithium-sulfur battery system.
Also, U.S. Pat. No. 5,961,672 suggests a mixed solution of 1M LiSO3CF3 and 1,3-dioxolane/diglyme/sulfolane/dimethoxyethane (50/20/10/20) as an electrolytic solution to improve lifetime and stability of a battery by coating a polymer film on a lithium metal anode.
In U.S. Pat. No. 5,814,420, an electrode containing active materials is brought into contact with both an ion conducting material and an electron conducting material such that the active material such as activated sulfur and/or polysulfide polymer can be almost completely used.
U.S. Pat. No. 5,523,179 discloses a lithium-sulfur battery comprising an active sulfur-based material, an ion conducting material and an electron conducting material in an anode.